Heat generation and management of molten material in an injection molding apparatus is important for ensuring the production of high quality molded parts. Heating of the molten material is typically accomplished by locating several electrically powered heaters adjacent to the flow channel of the machine nozzle, the mold manifold and the hot runner nozzle. Several different types of electrical heaters are available including coil heaters, band heaters, film heaters, heat pipes, induction heaters and cartridge heaters. The heaters are sometimes integrated or embedded into the nozzle housing in order to optimize the heat transfer to the molten material. Integrated electrical heaters are more expensive to manufacture and typically cannot be replaced without replacing the entire nozzle.
It is often preferable to use removable heaters because they are less expensive to manufacture and can be replaced without replacing the entire nozzle. A disadvantage of the known removable heaters in injection molding is that achieving efficient heat transfer between the heater and the nozzle can be difficult. Because the heater is a separate component, gaps can occur between the heater and the nozzle or manifold, any gap between these components reduces the efficiency of the heat transfer. The amount of contact between the heater and the nozzle or manifold must therefore be maximized. As a result, clamping solutions have been developed. Ideally, an optimum heater clamp would provide a good heat transfer from the heater to the nozzle irrespective of the actual temperature of the heater clamp. An ideal clamped heater would operate perfectly in hot conditions and would continue to operate perfectly regardless of temperature changes or variations from higher temperatures to lower temperatures. This means that the temperature fluctuation of the heater would not affect the clamping force between the heater and the nozzle.
A further disadvantage of known removable heaters is that they often require additional space to accommodate a locking mechanism. This is a problem in high cavitation molding applications where the space between the adjacent nozzle is minimized.
Referring to FIG. 1, a prior art clamp comprising a cylindrical heating sleeve 4 is shown. The heating sleeve 4, which includes heating elements 5 embedded therein, surrounds a nozzle body 6 to transfer heat thereto. The heating sleeve 4 includes an axial gap that provides a spring like characteristic. A clamping mechanism 7 having a screw 8 is provided for tightening the heating sleeve 4 about the nozzle body 6. The heating sleeve 4 is installed and clamped around the nozzle body 6 when the nozzle body 6 is in the cold condition. During regular operation, heat expansion causes the nozzle body 6 and the heating sleeve 4 to expand radially, as indicated by arrows 9. When the injection molding apparatus is turned off, the heating sleeve 4 and nozzle body 6 should return to their original size. This continuous heating and cooling of the heating sleeve 4 and the nozzle body 6 causes the contact between the heating sleeve 4 and nozzle body 6 to be reduced over time. This reduces the heat transfer between the heating sleeve 4 and the nozzle body 6. Therefore, it is necessary to readjust the clamping mechanism 7 on a frequent basis.
Several attempts have been made in the prior art to address this problem. The prior art solutions include several different clamping devices for exerting a compressing force on the heater in order to maintain contact between the nozzle body and the heater.
U.S. Pat. No. 4,268,241 discloses a removable annular heating element that is maintained in position by a nut. The nut is threaded onto a threaded lower portion of the nozzle near the nozzle tip.
U.S. Pat. No. 4,940,870 teaches an induction heating element for hot runner nozzles that includes a clamping sleeve having axial slots of various lengths.
U.S. Pat. No. 6,043,466 discloses a clamping sleeve that surrounds a heater. The clamping sleeve has a lower coefficient of thermal expansion than the heater and therefore causes the heater to be compressed against the nozzle when heated. The clamping sleeve may also be preloaded to exert a compressing force on the heater in the cold state.
U.S. Pat. No. 6,163,016 discloses a removable heater that is surrounded by a clamp. A pair of collars at opposing ends of the clamp are provided to compress the heater against the nozzle body.
U.S. Pat. No. 6,409,497 discloses a jacket-heating unit for a nozzle. The heating unit is surrounded by a sleeve that is flexible in the radial direction. A circular lock surrounds the sleeve and is rotatable between a released position and a clamped position. The sleeve and the circular lock include facing surfaces that have profiles that deviate from that of a cylindrical shell.
Achieving full contact between smooth heater surfaces and smooth nozzle or manifold body surfaces having different expansion coefficients is a difficult task particularly when the temperature of the heater cycles between hot and cold temperatures. As a result, the clamping heater devices of the prior art tend to be complex and thermally less efficient than expected. In addition, some skill and additional time is typically required to properly install the prior art devices.
It is therefore an object of the present invention to provide a removable heater for an injection nozzle or tubular manifold, which obviates or mitigates at least one of the above disadvantages.